Existing actuators for fin control on gun-launched projectiles are known, but are both complex and expensive. The requirement to withstand the acceleration forces, which typically range from 10,000 to 30,000 times the force of gravity, places very stringent demands on the actuators. Therefore, the designs are required to be extremely robust in order to withstand the loads induced by these accelerations. Existing actuators for fin control on gun-launched projectiles typically employ electric motors to drive the fins through a gear reduction system. These motors are either brush or brushless types that make several revolutions of the motor while moving the fin from one travel limit to the other. In the case of the brush type motors, there are substantial reliability issues with the brush systems due to the high acceleration loads and problems with corrosion resulting from long-term storage. The brushless types have reliability issues with rotor position sensing complexity.
U.S. Pat. No. 6,752,352 discloses an actuator system for controlling the external fins on a gun-launched projectile to control the flight path of the projectile. The actuator system includes an electric motor having a rotor and output shaft which is driven between travel limits that are less than 180 apart (less than 90 in either direction from a central rest position). Coupling from the motor shaft to the control shaft for the external fins is via a coupling between an eccentric ball on the motor shaft and an eccentric receptacle member on the fin shaft. As the angle of the motor shaft varies, the eccentric ball slides in a slot in the fin coupling member, causing the fin shaft angle to vary correspondingly. In another embodiment, the eccentric ball for controlling the fin shaft angle is mounted on a link arm that is coupled to the motor shaft, thereby permitting the motor to be mounted off the projectile axis and thus accommodating a shortened space in the projectile required for the actuator system and associated power supply. U.S. Pat. No. 6,880,780 to Perry et al. discloses a fin cover release and deployment system for gun -launched missiles, which uses a pyrotechnic actuator to drive actuator arms or a motor and rotating threaded shaft. The motor and rotating threaded shaft requires the use of an additional cover eject spring, which is not necessary in the pyrotechnic actuator.
Known deployment mechanisms for extending the fins in flight add complexity and reduce reliability of the projectiles, especially when stored for extended durations. Typically, the deployment mechanisms are pyrotechnics. Alternatively, the fins are deployed by a mechanical interface with the launcher, such as being retained by the launch tube walls, a an ejectable cover or being deployed by a lanyard, which effectuate release of the fins from the stowed position at a preset distance from the launcher. Pyrotechnics may be unreliable if stored for extended durations. Mechanical mechanisms involving the launcher are known to introduce drag and airframe instabilities. Ejectable covers require additional cover release springs and add additional complexity.